The present invention relates to a method of packing filler into the through-holes in a printed circuit board.
There are two conventional methods of packing filler composed of a conductive material into through-holes in a printed circuit board. In one method the conductive material is attached to a packing pin, and then is transferred to the inner wall of a through-hole from the outer peripheral surface of said packing pin by inserting said pin into a through-hole that has been bored in the printed circuit board. In the other method, the conductive material is packed into each through-hole on the printed circuit board through a printing silk screen that has open-work corresponding to the circuit design of the printed circuit board.
The former described packing method, which uses said pin system, requires a plurality of repeated operations to attach conductive material to the packing pins and then to insert those pins into their respective through-holes on the printed board in order to embed the material inside the through-holes. This method is inefficient and its operations are decidedly complicated because it is difficult to attach enough conductive material to each pin.
Moreover, in such operations, the conductive material must be heated for curing each time it is transferred to the inner wall of a through-hole from a packing pin. This inflicts much thermal damage on the printed circuit board's structural material and significantly deteriorates the board's dimensional stability.
In addition, when a printed circuit board has many through-holes bored, many pins must be used at once. Since pin-arranging design limitations make it usually impossible to use more than 1,000 pins, the area of a printed circuit board is limited in many cases to 300 mm.sup.2.
It is also difficult to mass produce, install, and operate so many pin jigs whose pins must be arranged to correspond to the through-holes of a printed circuit board.
When the latter described silk-screen method is used, the squeegee's pressure against the screen is not uniform. In addition, under the usual squeegee pressure, it is difficult to pack conductive material into each through-hole on the printed circuit board, and impossible to do so if the printed circuit board is thick or the diameter of the through-hole is small. In the present technical standard for packing conductive material, 0.7 mm is the minimum diameter of a through-hole when the board thickness is 1.6 mm.
In the two above-described conventional methods the poor fluidity of the conductive material has a large effect on the uniformity of packed material and tends to complicated operations. The usual countermeasure is to add solvent to the conductive material to make its fluidity more uniform. This remedial measure, however, causes gas bubbles to accumulate and cracks to develop in the through-holes as a result of heat generation and consequent solvent evaporation during the hardening of conductive material after being packed. This results in the possible failure of conductivity and difficulty in removing the solvent.
Thus, a higher-alcohol-based solvent with a high boiling point, and thereby a resistance to vaporization, is often used to retain desirable fluidity for a long duration. But this causes another difficulty. Namely, that the complete removal of such a solvent without generation of bubbles requires the vaporization of the solvent over a long period of time and at a sufficiently low temperature to prevent unremoved solvent from decreasing the reliability of the printed circuit board.
Another problem is that if filler packed in a through-hole protrudes and is hardened on the upper or lower surface of the substrate, the protrusion will have a harmful effect on subsequent treatment processes. For instance, press work using a die in the machining process during the manufacture of the printed circuit board will create a compressive force on the protrusion, and cracks or fractures will form in the conductive material filled in the through-hole and lessen the precision and reliability of the through-hole itself.